Thermodynamic constraints on methanogenic crude oil biodegradation

Area of Science:

• Geomicrobiology
• Environmental Microbiology

Background:

• Methanogenic degradation of crude oil hydrocarbons is crucial in subsurface petroleum reservoirs and contaminated anoxic environments.
• Several biochemical pathways exist for hydrocarbon conversion to methane, involving intermediates like H2, CO2, and acetate.

Purpose of the Study:

• To evaluate the thermodynamic feasibility of different hydrocarbon-to-methane conversion routes using a 'window of opportunity' concept.
• To reconcile theoretical thermodynamic favorability with observed microbial processes in petroleum environments.

Main Methods:

• Thermodynamic analysis of five proposed methanogenic degradation pathways.
• Development and application of the 'window of opportunity' concept to assess pathway feasibility under varying conditions.

Main Results:

• Oxidation of alkanes to acetate alone, linked to acetoclastic methanogenesis, theoretically offers the largest 'window of opportunity'.
• However, field evidence suggests acetoclastic methanogenesis is inhibited in petroleum-rich environments, with CO2 reduction being dominant.
• Under biological constraints, oxidation of alkanes to acetate and H2, linked to syntrophic acetate oxidation and CO2 reduction, presents a greater thermodynamic window than complete alkane oxidation.

Conclusions:

• The thermodynamically most favorable pathway (alkanes to acetate alone via acetoclastic methanogenesis) is not always the predominant process in situ.
• Syntrophic acetate oxidation coupled with methanogenesis from CO2 reduction is identified as the most probable methanogenic route for hydrocarbon degradation in petroleum environments, aligning with field observations.